KEY BISCAYNE, Fla. -- When professor Serena Williams was asked to grade Wednesday's first set against China's Li Na, she did not hesitate.
"I would give myself like an 8 percent out of 100, which is failing," she said, drawing laughter.
How about the entire quarterfinal match at the Sony Ericsson Open?
"The whole match is an F," Williams said. "But it is what it is. I'm honestly glad to get through it. I feel like I wasn't as upbeat and spirited as I normally am."
Williams, oddly laconic and seemingly disinterested, fell into a 0-5 hole to Li on a breezy afternoon. After winning the next four games, she lost the first set, but thrashed Li in a second-set tiebreaker, winning seven of eight points. After Serena won five straight games in the third, the final was: 4-6, 7-6 (1), 6-2.
That Serena had to work so hard was more than mildly surprising. This is Miami, after all.
Room service is an unpleasant fact of life in professional tennis, the price paid for all those frequent-flier miles. But here in Miami, the Williams sisters are beneficiaries of door-to-door service of a more pleasant kind. The Sony Ericsson Open, the most celebrated non-Grand Slam event, may soon be rechristened The Williams Open.
"Every good gift comes from above, right?" Venus said the other day. "That's what I attribute it to."
There may be slightly more to it.
While most of the world's elite tennis players are grinding through the dry heat of the desert in Indian Wells, an event the sisters have boycotted since 2001 after some alleged ugly incidents, they sleep in their own beds, eat familiar food and frolic with their three dogs -- Harold the Havanese, Lorelei the Maltese and a Jack Russell Terrier named, well, Jack.
When everyone else must make the 3,000-mile journey to Miami, the Williams' leave their $1.8 million home in Palm Beach Gardens and navigate the modest 70-mile drive south. And while Serena stays in a local hotel with most of the sprawling family entourage, Venus keeps a nearby condo and splits her time between that and the hotel.
The combination of local knowledge and a heightened comfort level, coupled with the fresh tank of gas that skipping Indian Wells affords them, has resulted in nothing less than dominance in Miami. Serena has won five of the last seven tournaments here, and adding Venus' previous run of three-for-four, they have won a collective eight of 11 Sony Ericsson titles.
Only one of them will play in this year's final, because the sisters find themselves opponents in Thursday night's top-half semifinal.
On Wednesday night, Venus won six of the first seven games against Iveta Benesova in 25 minutes. Although she dropped the first three games of the second set in a brief, Serena-like lapse, Venus rallied to close out Benesova 6-1, 6-4.
"I enjoy playing her because we challenge each other the most," Venus said of Serena. "I walk out there knowing it's not over until it's over, so I have to beat her.
"I think we are the best in the world."
The depth and breadth of the Williams' success at the lush 30-acre Tennis Center at Crandon Park underlines their enormous staying power in a women's game marked by injury and attrition.
When 17-year-old Venus broke through here 11 years ago, it was the first major title for the Williams sisters. It was quaintly called the Lipton Championships, and Venus beat Martina Hingis in the semifinals and -- wonder of wonders -- Anna Kournikova in the final.
Since then, only three other women have encroached on the Williamses marked territory -- Hingis (2000), Kim Clijsters (2005) and Svetlana Kuznetsova (2006) -- and there is a reasonable chance they will add a ninth family title. To put that in perspective, since Earl "Butch" Buchholz founded the tournament in 1985, the two best résumés on the men's side combined for nine titles -- and they were named Agassi and Sampras.
And although people have routinely criticized the sisters for playing too few tournaments and allowing themselves the pleasures of life outside tennis, give them credit for staying the course and maintaining their games at a consistently high level.
A decade ago, when Venus won her second Miami title, Hingis and Lindsay Davenport were trading places at No. 1. Jennifer Capriati rose to the top before Venus, then Serena reached No. 1 for the first time, in 2002. They were followed by Belgians Kim Clijsters and Justine Henin, then Amelie Mauresmo and Maria Sharapova and, most recently, by the Serbians, Ana Ivanovic and Jelena Jankovic. Victory at the Australia Open put Serena back on top for the first time in 5½ years.
And now, the Williams will meet for the 20th time in a WTA match; this is the fourth occasion in Miami.
Venus holds a scant 10-9 edge and has won the last two matches, albeit in three sets.
"I'm always going to be up for V, because she's really playing well and always playing me super-tough," Serena said. "I feel like playing her, it's like I have to automatically be on a different level, because she's already on a different level.
"Her balls are harder and her serve is way bigger. It's like I have to be there. And it's fun. It's super-fun to hit, like, these serves that are like 120. It's frustrating, but fun at the same time."
Serena is currently ranked No. 1, while Venus is No. 6. They are a combined 35-3 this season, and their Miami victory total is now up to an imposing 93.
Through the years, the sisters usually have been reticent to discuss their sibling rivalry in any kind of personal detail. On Wednesday, they seemed more relaxed and free.
Venus appeared surprised to learn she had won the last two matches between them.
"Have I?" she asked, coyly. "Oh, yeah! Yes. Hopefully, that will mean something tomorrow."
Later she added, "I don't want to knock her off her ranking, but I do want to win."
Greg Garber is a senior writer for ESPN.com.